Lenses which have a gradient of refractive indices (GRIN lenses) have been known for some time. These lenses have numerous uses in the optics, optical fiber and solar technology industries and are useful in designing compound lens systems using a single, integral lens or a reduced number of lenses. A GRIN lens can exhibit a change of refractive index along its optical axis or bi-directionally (both orthogonally radial to and along the optical axis). More complex GRIN lenses, which have changes in refractive index in three dimensions, are also known.
An example of a lens which has a chosen gradient in its index of refraction both orthogonal to and longitudinally along an optical axis is illustrated in U.S. Pat. No. 4,883,522 to Hagerty. Other examples of GRIN lenses can also be found in U.S. Pat. No. 4,929,065 to Hagerty and in the documents referred to therein. In particular, the '065 Hagerty patent, the disclosure of which is incorporated herein by reference, discloses a method of manufacturing a GRIN lens which has a large change in index of refraction over a significant dimension along only its optical axis. The method disclosed in this patent requires that a number of glass wafers each having a different refractive index, be stacked on top of one another. The stack is then heated to above the fusion temperature of the individual wafers which fuse together to define a contiguous unit of optical material. The fused stack can then be ground to form the defined GRIN lens.
This method, when used correctly, can produce a good quality GRIN lens with an accurately defined gradient in refractive index. The method does, however, have the disadvantage that it is a very expensive and time consuming to prepare the individual wafers required. This is partly because the wafers must be accurately ground to the desired thickness and must have very smooth surfaces to reduce the amount of air bubbles which would otherwise be trapped between the wafers as they are stacked to form a block. The required cutting, grinding and polishing can, in extreme cases, result in the loss of as much as 30% to 50% of the glass.
In an alternative prior method of manufacturing GRIN lenses, two glasses are selected, the indices of which represent the end members of the gradient profile desired. Each is ground to a powder and then mixed together in suitable proportions calculated to create a series of mixtures, each of which when fused has the index required for the gradient profile. Each mixture is then carefully and successively layered into a platinum alloy mold. The mold with glass powder is placed into a furnace and slowly heated to fusion temperature, then held at a predetermined temperature for diffusion and slowly cooled.
This method has the disadvantage in that the two glass types in the powder usually have different densities, with the higher density glass having a lower melting temperature. As the glass particles start to melt, the higher density glass melts first and tends to sink to the bottom of the mold and the less dense glass particles tend to move up toward the surface of the molten denser glass before they melt. As a result, separation of glass types within layers occur and mixing between layers occurs. In extreme cases the denser and less dense glasses separate into substantially two different layers, resulting in a body made up of essentially two layers having different refractive indices, as opposed to the continuous gradient in refractive index required. Because of this effect, it is very difficult to accurately control the gradient of refractive index using this process.
Furthermore, it is very difficult to remove small bubbles of air that are trapped in the powder as it melts. This means that the final piece of optical material could have unacceptably large defects as a result of trapped air.
Accordingly, a need has arisen for a process of manufacturing a GRIN lens, which has a gradient of refractive indices along its optical axis, which does not have the above disadvantages. This process should be able to produce a GRIN lens with a similar refractive index profile to the "wafer process" in the Hagerty patent above, produce a lens of high optical quality, and must be reproducible.